1. Technical Field of the Invention
The present invention relates generally to mobile Broadband Wireless Access (BWA) communications systems. The invention more particularly relates to a MAC and TCP coordination approach in mobile BWA communications systems which improves TCP performance in the mobile environment where temporary link disconnections caused by handover (HO) are frequent and unpredictable.
2. Description of Related Art
The Transport Control Protocol (TCP) has not proven to be efficient in mobile wireless networks where temporary disconnections caused by handover (HO) may unpredictably occur. There is a need for an optimized TCP for the mobile environment in association with the incorporation of a mobility function in an IEEE 802.16e network. Significant research efforts have been made to propose enhancements to TCP performance in mobile conditions. However, these proposals suffer from numerous drawbacks. For example, the proposals may require unacceptable changes to the existing communications infrastructure. Additionally, the proposals often ignore the common scenario that the mobile station may act as both a TCP sender and a TCP receiver.
An acceptable solution to the issues surrounding the use of TCP in a mobile environment must recognize that the mobile station may act as both a TCP sender and a TCP receiver. Additionally, a solution to the problem must minimize changes to the infrastructure and more preferably restrict changes to only the mobile station while using existing TCP protocol at the remote sender or receiver.
BWA is emerging as an integral part of the next generation (4G) wireless access infrastructure. BWA targets providing high-speed wireless access to data networks. This allows users to stay connected to the Internet, to use a variety applications, and to access digital media simply by using wireless handheld devices connected through omnipresent access portals (APs). One particular BWA technology is being standardized through the IEEE 802.16 standard to offer high data rate wireless “first-mile/last-mile” broadband access in a Metropolitan Area Network (MAN). At present, the IEEE 802.16 standard family consists of four amendments or updates: 802.16, 802.16a, 802.16d (802.16-2004[1]), and 802.16e (802.16-2005[2]). To take advantage of the inherent mobility of wireless media in BWA systems, the recently released 802.16e specification defines the mobility operations in the 2-6 GHz licensed bands and promises to support mobility at mobile station speeds of up to 70-80 mph.
IEEE 802.16e is a standard for BWA that supports intra-domain mobility. This standard may fill the gap between fixed wireless, local area networks and mobile cellular systems. To enable users to make use of a wide-area network through an access network when mobile, the IEEE 802.16e standard defines the handover (HO) process in which a Mobile Station (MS) migrates from the air-interface provided by one Base Station (BS) to the air-interface provided by another BS.
The Medium Access Control (MAC) layer of the protocol stack has a HO process that comprises two main stages: the first stage is the HO pre-registration phase and the second stage is the real HO phase. During the HO pre-registration phase, the target BS is selected and pre-registered with the MS. However, the connection to the currently serving BS is maintained during the pre-registration phase and packets continue to be exchanged with the currently serving BS. In the real HO phase, the MS releases the currently serving BS and re-connects with the target BS. Packet exchange then proceeds with respect to the target/newly serving BS.
According to the IEEE 802.16e standard, either the MS or the currently serving BS may initiate the HO pre-registration phase. After an MS or a currently serving BS initiates the HO pre-registration phase, the currently serving BS may negotiate the intention to perform a HO with its neighboring BSs (through exchanged backbone communications). These negotiations concern whether each of the neighboring BSs possesses the capability to serve the MS. The currently serving BS may further notify the selected neighboring BS of the impending HO.
FIG. 1 shows an example of MAC layer HO process initiated by the MS. The MS engages in a process to scan neighboring BSs for better communication than with the currently serving BS and may make a decision that a HO is needed. A HO request (MOB_MSHO-REQ message) is then sent from the MS to the currently serving BS (where the request may recommend one or more candidate target BSs). The currently serving BS (in this case BS#1) then sends an HO-notification message to each of the candidate target BSs (for example, BS#2 and BS#3). This message includes an identification of the MS and information concerning the communication service to be provided (BW, QoS, connection parameters). Each notified target BS evaluates whether service can be provided and responds with an HO-notification-response message which includes an acknowledge (ACK) or not acknowledge (NACK) indication with respect to accepting a possible HO of the MS. More specifically, the target BS indicates whether it can (or cannot) participate in the HO and service the MS. This message may further qualify an acknowledgement concerning a class of QoS that the target BS can support. After that, the serving BS selects one or more of the target BSs as being acceptable for the HO, and sends a handover response (HO-RSP) message to the MS. This message includes an identification of each acceptable target BSs. The MS then makes a target BS selection from the acceptable list and transmits an HO-IND message to the serving BS. This message identifies which of the BSs on the acceptable list has been selected by the MS for the HO. This message further provides the serving BS with a final indication that it is about to perform a real HO. The serving BS then releases communication with the MS, and the MS synchronizes with the selected target BS in a manner well known in the art (see, Fast_Ranging_IE, RNG_REQ and RNG_RSQ). After re-authorization and re-registration are completed with respect to the selected target BS (see, complete initial network entry after HO), the communication service flows with the MS can be re-established through the newly serving BS.
Since the IEEE 802.16 standard for BWA is expected to be an attractive alternative to the use of traditional wireline services through cable modem, xDSL and/or T1/E1, the users of this potential wireless system will require transparency with respect to the distinct link characters of the wireless communication while still being able to use the same applications as they do in wired networks. This means that the dominant transport protocol in a wired network, TCP, remains a vital component of the transport layer in the IEEE 802.16 network.
However, with the introduction of mobility in the IEEE 802.16e standard, a MS handover in the MAC layer may lead to temporary disconnection and packet loss in the TCP layer. Because the TCP protocol was originally developed for the wired networks, the TCP protocol has no idea about the possibility or implementation of HO in the MAC layer. Once packet loss occurs due to HO, the TCP layer mistakenly concludes that the network is congested and immediately initiates the known TCP congestion control algorithm. Hence, during the temporary disconnections caused by the HO of a MS, the TCP layer reduces its congestion window to a minimum value. After the link is reconnected following the completion of HO, the TCP layer then invokes the slow-start algorithm. This is a relatively slow process in the TCP layer during which the congestion window returns to its previous value (i.e., before the HO). Therefore, a period of time may be wasted with respect to communications after the MS reconnects with the target BS following HO. As a result, the temporary disconnections caused by HO of the MS degrade TCP throughput and radio resource utilization.
Known attempts to address some of the problems noted above include solutions proposed by: (1) Rhee, et al., “MTCP: Scalable TCP-like congestion control for reliable multicast,” Proc. IEEE INFOCOM, 1999, pp. 1265-1273; (2) Balakrishnan, et al., “Improving reliable transport and handoff performance in cellular wireless networks,” ACM Wireless Networks, pp. 469-481, December 1995; and (3) Goff, et al., “Freeze-TCP: A true end-to-end TCP enhancement mechanism for mobile environments,” Proc. IEEE INFOCOM, 2000, pp. 1537-1545. These solutions specify two classes of TCP optimization approaches in order to improve TCP performance in a mobile environment: the first is TCP optimization based on intermediary (such as BS) assistance, such as described in the Rhee and Balakrishnan references; and the second is end-to-end TCP optimization without the involvement of any intermediaries such as Freeze-TCP as described in the Goff reference.
The first class of solution resides on an intermediate host (such as the base station). It requires the intermediate host to cache packets from the sender and to inspect their TCP headers. Using information snooped from the communications, if the intermediate host determines that a packet has been lost, it retransmits a buffered copy to the mobile node (which is intended to be a local retransmission over one or multiple links). In this process, the intermediate host maintains its own timers for retransmissions of buffered packets, implements selective retransmissions, etc.
The second class of solution does not involve any intermediaries in flow control. Instead, it is an end-to-end scheme applying only to the circumstance where the MS is a TCP receiver. The main idea is to move the onus on signaling an impending disconnection to the client. A mobile node can certainly monitor signal strengths in the wireless antennas and detect the need for an impending HO. In certain cases, the mobile node might even be able to predict a temporary disconnection (if the signal strength is fading, for instance). In such a case, the mobile node can specify a zero window size and thus force the sender TCP into the Zero Window Probes (ZWP) mode which would prevent it from dropping its congestion window as a result of lost packets due to the handover.
As described above, the available TCP optimization approaches attempt to improve TCP performance during the HO process within the TCP layer. However, these solutions have some weaknesses. The first class of solution requires the BS to monitor the TCP traffic and assist in performance enhancement. Thus, this solution demands modification at both the BS and the MS. The BS should parse all the packets routed through it and then decide whether it is a TCP packet. This solution is not compatible with the existing infrastructure of the IEEE 802.16e standard. Even further, if the IP payload is encrypted, then this approach cannot work.
The second class of solution focuses on performance enhancement in the case where a MS acts as a TCP receiver. This solution ignores the common scenario that a MS may also act as a TCP sender due to the high data rate offered by the BWA system.
A need accordingly exists in the art for a better solution which would minimize the adverse impact of a HO on TCP performance, while being easy to implement and administer.